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Testing a claimed heat-pump efficiency
The air in the building gains 400 J for every 100 J of energy transferred from the mains electricity supply to the compressor. An advertisement claims that the heat pump system has an efficiency of 400%.
The air in the building gains 400 J for every 100 J of energy transferred from the mains electricity supply to the compressor. An advertisement claims that the heat pump system has an efficiency of 400%. Explain why the advertisement is not correct.
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Marking points
- 1 State that the advertisement ignores energy input transferred from the surrounding or outside air.
- 2 Explain that the total energy input to the heat pump is greater than just the energy supplied by the mains electricity.
- 3 State that efficiency must be less than 100%.
Full-mark answer
The 400 J gained by the air is not produced from only the 100 J supplied by the mains. The heat pump also takes in energy from the surrounding air, so the total energy input is greater than the electrical input alone. Therefore the efficiency cannot be 400%; the efficiency must be less than 100%.
Why this answer loses marks
The advertisement is correct because .
This repeats the advert's incomplete calculation and ignores the extra energy input from the surrounding air.